1. Field of the Invention
This invention relates to an artificial stem unit, which is used as the artificial stem unit for coxa in the case where the coxa of human body suffers from functional disorders in the joint region.
2. Prior Art
An example of conventional artificial heads(stems) of the coxa is illustrated in FIGS. 5 and 6.
In the case mentioned in FIG. 5, bone cement 13 is injected into the cavity of bone marrow to fix a stem 11 inserted by using no centralizer. The numeral 14 represents the cancellous diaphysis and the numeral 15, the femur.
In the case mentioned in FIG. 6, there is inserted beforehand, into the cavity of marrow of a femur forming the diaphysis 14, a centralizer 16 made of bone cement and having a convexity 16a. Then a stem 17 is set within the cavity of the femur marrow and the bone cement 13 is injected into the marrow cavity by means of a cement injector. Thereafter, the stem 17 which is provided at the tip thereof with a concavity 17a conformable to the convexity 16a of said centralizer is pushed into the diaphysis so as to be fit to the centralizer. In this way, the stem may be appropriately positioned within the femur by means of the centralizer.
But the aforementioned conventional device have the following disadvantages.
(1) As the concavity 17a provided at the tip of the stem is shallow, the stem is often fixed in such manner that the central axis A of the joint is slightly out of the central axis B of the stem, unless the stem is skillfully inserted in the diaphysis. Thus, the stem may not sometimes be appropriately positioned within the femur.
(2) When it takes much time to insert the stem 17 into the cavity of the bone marrow 14 for appropriately fitting the stem to said centralizer, the bone cement injected in advance would start to harden and the surgical operation would become uncontrollable. Thus, this type of operation would require a considerable degree of skill.
(3) When the stem 17 is inserted, air would enter in between the stem and the bone cement 13. It may not be possible to remove the air thoroughly. Thus, the strength of adhesion would be lowered.
(4) When the stem is inserted in such manner that it comes off the central axis, the bone cement would not be thoroughly filled up and the pressure may not be evenly placed upon the bone cement.
As described above, there is a problem that the stem may not be accurately positioned, in the stem replacement operation employing the combination of a stem with concavity and a centralizer with small convexity.